FIG. 15 is a left side view of a prior art bulldozer 1 equipped with a blade (earth-moving plate) 50. The bulldozer 1 has a pair of elongated frame members 3, each of which has one end pivotally mounted on a respective lateral side of a body 2. The blade 50 is pivotally mounted to the distal ends of the frame members 3 such that it can swing in the longitudinal direction of the bulldozer 1. The blade 50 is also pivotally coupled at its transverse opposite ends to intermediate portions of the frame members 3 by respective pitch hydraulic cylinders 4A (shown on the left side of the body 2) and 4B (located on the right side of the body 2), and is also pivotally coupled to the body 2 by a lift hydraulic cylinder 5. A front panel 51 of the blade 50 is formed so as to have a curved or concave surface in a vertical plane, and a blade edge member 52 is fitted to a lower end of the front panel 51 while extending substantially tangential to the curved surface of the front panel 51. By operating the lift hydraulic cylinder 5 to extend or contract, the blade 50 is raised or lowered as indicated by the arrow A. Also, by operating both of the pitch hydraulic cylinders 4A and 4B to simultaneously extend or to simultaneously contract, the blade 50 is pitched in the longitudinal direction as indicated by the arrow B. Further, by operating only one of the pitch hydraulic cylinders 4A, 4B to extend or contract, or operating one to extend and the other to contract, the blade 50 can be tilted to the right or to the left, as indicated by the two broken rectangles in FIG. 16.
In another prior art bulldozer as proposed in Japanese Utility Model Laid-Open No. 3-50646, for example, a blade is operated in the lift, tilt and pitch modes by one control lever and by one operation changeover switch between the tilt and pitch modes. While an edge angle .alpha. (the angle between the front face of the blade edge member and a horizontal line as shown in FIG. 15) of the blade, formed when the blade is placed on the ground, is suitably about 55.degree. for the nature of ground and work conditions in the usual case, it can be adjusted to the extent of .+-.5.degree., depending on differences in the nature of ground and work conditions.
On the other hand, looking at the balance of forces when a bulldozer is performing earth-moving work, as illustrated in FIG. 17, the traction force F.sub.2 must be greater than the earth-moving resistance F.sub.1 and the vehicle's driving force F.sub.3 must be greater than the traction force F.sub.2. Specifically, assuming that the weight of earth 6 being moved by the blade is G, the friction coefficient of the moving earth 6 with respect to the ground surface is .mu..sub.1, the weight of the bulldozer 1 is W, the friction coefficient of the bulldozer 1 with respect to the ground surface is .mu..sub.2, the engine torque is T.sub.0, the speed reducing ratio is .rho., and the radius of a driving wheel is R, the relationship of EQU F.sub.1 =G.times..mu..sub.1 &lt;F.sub.2 =W.times..mu..sub.2 &lt;F.sub.3 =T.sub.0 .times..rho./R
is required to be satisfied.
In the past, therefore, the amount of work performed by the bulldozer has been increased by scaling up the size of the bulldozer, increasing the engine output, and enlarging the capacity of the blade. When it is desired to double the amount of earthwork, for example, this is achieved by manufacturing a bulldozer in which the engine output is substantially doubled and the vehicle weight is also substantially doubled. Thus, a series of bulldozers with different capabilities has been manufactured in accordance with the above concept. Further, the capacity of a hydraulic pump for driving working equipment in the prior art bulldozer is usually set to correspond to the capacity required for the lift hydraulic cylinder which is used to raise or lower the blade.
However, in any attempt to manufacture a bulldozer capable of effecting twice the amount of earthwork that is achievable by the largest bulldozer in the same series, technical problems, such as relating to vehicle weight and materials, must be solved in order that the stresses exerted on the frame members and other parts of the body and the life of the driving apparatus can be kept within allowable values. In solving the above technical problems, the production cost of the bulldozer is increased at a rate greater than linearly proportional with respect to the amount of earthwork. Akio Terai, Herb Aoki, and R. H. Stanage, of Komatsu America Corp., presented a report in SAE Paper No. 790902 on those technical problems. Stated otherwise, although there is a demand for bulldozers capable of moving a great amount of earth, such demand has not been met because of technical and economic difficulties.